Since the inception of machineguns in the late 19th century, millions of self powered firearms firing high pressure smokeless powder cartridges have been manufactured. All high pressure small arms weapons, with a few experimental exceptions, have been provided with locking mechanisms of several basic types. Conventional locked operating mechanisms are expensive because they must be manufactured to close tolerances from high strength materials. Conventional locking mechanisms must be robust and capable of closely supporting the cartridges which must also be manufactured to close tolerances. Locked weapons and their cartridges must be manufactured to close tolerances in order to maintain “headspace” within workable limits while subjected to pressures in excess of 50,000 pounds per square inch.
Practically speaking, headspace is the distance between the locked bolt and the base of a cartridge seated fully forward in the chamber of a weapon. If headspace is excessive, then when the cartridge is fired and while the wall of the cartridge case is seized against the chamber wall, the base of the cartridge can move excessively rearward before contacting the locked bolt. In this event, the cartridge case head can be ripped off the body of the cartridge case resulting in a ruptured cartridge case which usually disables the weapon and can cause severe injury to the shooter. If headspace is insufficient, then the cartridge is too long to fit into the chamber resulting in the failure to lock, and often wedging the cartridge tightly within the chamber, also disabling the weapon.
The high cost of providing close dimensional tolerance in weapon mechanism parts involved in locking has come to be taken as a matter of course in the small arms community.
Conventional cartridge cases are provided with extraction rims and grooves for removing unfired cartridges or fired cartridge cases from the weapon chamber. Conventional extraction rims and grooves are necessarily located behind the rear of the barrel in order to permit access of the extractor to the extraction rim and groove. The primer of a conventional cartridge is located in the rear of the cartridge with the base of the primer flush with the base of the cartridge case. This means that the primer is actually located outside of and behind the rear of the chamber. Therefore the cartridge case around the primer pocket provides the sole support for radial firing pressure. This means that the safe weapon operating pressure with conventional cartridge cases is limited by the strength of the cartridge case head rather than by the strength of the weapon itself, regardless of how strong the weapon breech.
Conventional high pressure bottle necked cartridge cases are not suitable for employment with simple blowback operating systems for two main reasons. First, high pressure cartridge cases cannot tolerate rearward movement of their heads while the case walls are seized in the chamber. Also, since the purpose the enlarged base diameter of bottle-necked cartridge cases is to provide large volume, this means the pressure/area for a blowback operated high pressure bottle neck-necked cartridge would be prohibitively large for an acceptable bolt mass.